The present invention relates generally to a method for producing core protrusion relative to the cladding in an optical fiber mounted in a ferrule of a fiber optic connector. The present invention also relates generally to a fiber optic connector which includes an optical fiber having such core protrusion.
Optical fibers are widely used in a variety of data transmission applications including, primarily at the present, the telecommunications industry. Because optical fibers transmit voice and other data far more rapidly and efficiently than copper wire, the demand for optical fibers is continuing to increase. For example, optical fibers no longer serve merely as the medium for long distance signal transmission but are increasingly routed directly to the home or, in some instances, directly to a desk or other work location to network computers. These fiber optic networks incorporate a number of remateable connectors instead of permanent splices in order to provide flexibility in revising or upgrading the networks. The remateable connectors generally include an optical fiber which is held in a 2.5 mm zirconia or stainless steel ferrule by an adhesive.
Because these remateable connectors are a source of reflectance, efforts have been focused on improving the relative geometry between the optical fibers and the ferrule in the connectors in order to improve connector reflectance. Due to the small size of the optical fibers, maintaining exact tolerances, generally measured in terms of microns (.mu.m) or nanometers (nm), is very critical but extremely difficult to do.
Present methods of processing the ends of the fiber optic connectors to achieve a desired end surface geometry include mechanical polishing, including grinding, and laser polishing are well known. Generally, the mechanical polishing methods disclose polishing the entire ferrule together with the optical fiber. For example, U.S. Pat. No. 5,007,209 to Saito et al., U.S. Pat. No. 4,905,415 to Moulin, U.S. Pat. No. 4,492,060 to Clark, and U.S. Pat. No. 4,272,926 to Tamulevich disclose mechanical polishing methods. In addition, commercial machines are available for mechanically grinding or polishing fiber optic connectors from Seikoh Geiken or others. Polishing pads, films, and slurries are also commercially available.
The surface of the polishing pads or films are made of various types of material for use with particular applications as is well known in the art. For example, carborundum film is used for removing adhesive from the fiber optic connector and for convex forming. Diamond films are used for convex forming, rough polishing, and medium polishing. Aluminum film is used for fine polishing. Polishing time varies from 1 to 1.2 minutes for adhesive removal and convex forming to 0.4 to 0.5 minutes for final polishing. As is well known in the art, applying the proper amount of pressure on the ferrule during polishing is important to obtain the desired convex surface on the end of the ferrule.
Standard published mechanical polishing steps include removing excess adhesive used to hold the optical fiber in place, along with any excess optical fiber, followed by convex forming which is the most important step in the mechanical polishing process. The end of the ferrule is inspected after convex forming to ensure proper convex formation of the end face of the ferrule. Next, additional polishing using diamond film and distilled water is performed. Final polishing is then accomplished using aluminum film and distilled water. The polished end of the ferrule is then inspected with a 100 power microscope to verify that the mechanical polishing has been properly completed, i.e. that a relatively smooth convex surface has been formed on the end of the ferrule with the optical fiber at the center or apex of the convex end of the ferrule.
To further improve fiber optic geometry, other methods of polishing the optical fiber and the connector have been developed which focus on the ferrule/fiber geometry, i.e, the relative position of the end of the fiber and the end of the ferrule, including polishing with a laser as disclosed in U.S. Pat. No. 5,226,101 to Szentesi et al., and U.S. Pat. No. 5,317,661 to Szentesi et al. In particular, a laser beam is directed to the end of the optical fiber in order to vaporize some of the glass at the end thereby reducing reflectance of the optical fiber by providing a smooth surface at the end of the fiber. U.S. Pat. No. 5,317,661 discloses a method of mechanically polishing the fiber optic connector followed by laser polishing the end of the optical fiber in the fiber optic connector. According to this method, the light beam from a CO.sub.2 laser operating in a low duty cycle pulsed mode of operation is repeatedly directed on the end of the optical fiber and the fiber optic connector. Repeated pulses from the CO.sub.2 laser directed to the optical fiber vaporize the end of the optical fiber while avoiding bulk melting of the end of the optical fiber.
Additionally as disclosed in U.S. Pat. No. 5,421,928 to Knecht et al., a laser may be used to prepare the end of a fiber optic connector having an optical fiber held therein by removing a portion of the optical fiber tip, both core and cladding, projecting beyond the connector end a predetermined distance. Excess adhesive may also be removed from the end of the connector by this process.
While mechanical polishing followed by laser polishing generally provides acceptable fiber optic end face geometry, potential disadvantages exist. Applicants have discovered a principal disadvantage of the above-mentioned two-step, mechanical/laser process is that the polishing steps may cause the core of the optical fiber to be recessed below the cladding of the optical fiber thereby creating an air gap between the cores of the optical fibers which increases reflectance in mated connectors. In some cases, a recess or undercut may be formed in the core itself. First, mechanical polishing may cause undercutting of the core of the optical fiber in the connector ferrule. Second, over processing of the fiber with a laser may also result in preferential removal of material from the core of the optical fiber itself which may create an air gap from between about 1-5 nm or more between the cores of mated connectors. As a result of these disadvantages, present connector reflectance values after mechanical and laser polishing run between about -60 to -65 dB.
Therefore, while methods exist for polishing optical fibers in fiber optic connectors, they do not meet the increasing demands for fiber optic connectors with improved return loss performance, i.e., lower reflectance. By improving return loss performance, the rate and accuracy of data transmission may be improved. Present mechanical and laser methods for polishing optical fibers have been optimized to the extent that little improvement in return loss performance can be achieved using these methods alone. As a result, other methods for polishing the end of the optical fiber mounted in a fiber optic connector must be developed to improve return loss performance of these remateable connectors.